1. Field of the Invention
The present invention relates to a device for controlling a DC motor driven diaphragm pump, and more particularly to a device for controlling discharge of a DC motor driven diaphragm pump, which is used as a metering injection pump.
2. Description of the Related Art
An electric motor driven diaphragm pump has been shown in the prior art. An electric motor used as driving device for a diaphragm pump is commonly a stepping motor or a DC motor (Direct Current motor). When a stepping motor is used, discharge of the pump is controlled by means of controlling a rotation speed of the stepping motor by modifying frequency or duty ratio of applied pulses to the stepping motor. Although discharge of the pump is accurately regulated by the stepping motor, as shown in FIG. 7 depending on the duty ratio of pulses, discharge of the pump is changed so much that it is not applicable to a diaphragm pump for small amount metering. Furthermore, a stepping motor and a pulse frequency modulating device or a pulse duty control device are expensive and the weight of these devices are heavy. In FIG. 7, the relationship between a rotation speed of a stepping motor and discharge of a diaphragm pump is illustrated in the case of setting a pulse width(PW1) at 40 ms, 100 ms and 200 ms.
In the case of using a DC motor for driving a diaphragm pump, direct current at a constant voltage is applied to the DC motor to be rotated at constant speed, thereby the diaphragm pump discharges continuously constant amount of fluid. A flow control valve is required to be provided in a line after the discharge port of the diaphragm pump for metering a amount of fluid. Moreover, when a DC motor runs continuously, temperature of the motor vises a large amount as shown on the curve A in FIG. 6. The curve A illustrates changing temperature of DC motor when runs at 3,600 rpm (applied 2V DC).
Another controlling device of a DC motor as a actuator a diaphragm pump is to regulate a rotating amount of the DC motor by application of pulses. When applying pulses, a DC motor rotates intermittently and pumping pressure of a diaphragm pump is controlled by varying applied pulse voltage and discharge per a pumping cycle is regulated by modulating duty ratio of applied pulses. In FIG. 6, the curve B shows temperature of a DC motor in this case, the temperature of the motor is not so high but an overshoot at rising and falling period of a pulse (as shown in FIG. 6B) is repeatedly impressed to the DC motor, generating a spark at the commutator of the motor and deposit carbon in a brush contact plain of a commutator. This results in a reduction of the service life of the DC motor.